Efficient two-photon-sensitized luminescence of a europium(III) complex.

The luminescence of lanthanide(iii) coordination complexes is unique in their high color purity, long lifetimes, and insensitivity to environmental quenching. These advantages have attracted intensive research efforts in the development of photosensitized and electrochemically driven luminescent materials and luminescent biolabels. Whereas the poor absorptivity of lanthanide ions owing to the parity-forbidden nature of the inner-shell f–f transition is readily overcome by the introduction of chelating chromophores with large extinction coefficients, it has long been a challenge to extend the wavelength range of photosensitization which is inherently hampered by the restriction of ligand-to-metal excitation energy transfer (EET) from the triplet excited state of the ligand. Very recently, we successfully extended the sensitization wavelength up to 460 nm by incorporating a novel ligand, 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine (dpbt), into a europium(iii)–thenoyltrifluoroacetonato (tta) complex, [Eu(tta)3dpbt] (Scheme 1). The luminescence of the Eu ion in this complex, with a quantum yield of 0.52 (lem = 614 nm), was sensitized by efficient EET from the lowest singlet excited state (S1) of dpbt to the Eu center. The N,N-diethylaniline group in dpbt functions as an electron donor (D) and the dipyrazolyltriazine moiety acts as an electron acceptor (A). Therefore, dpbt is a polar chromophore that shows D–A character, which is known to be the structural basis for an efficient two-photon absorber. As a newly emerging frontier with important application prospects, two-photon excitation (TPE) of a light-harvesting chromophore and subsequent EET to another functional chromophore in close proximity has been brought into practice by the use of femtosecond-pulsed laser sources. Recently, the TPE–EET strategy has found an increasing number of applications. For instance, in photosynthetic pigment–protein complexes, two-photon-excited carotenoids transfer excitation energy to fluorescent (bacterio)chlorophylls—this was used to detect the optically forbidden excited states of carotenoids which play key roles in the photochemistry of photosynthetic carotenoids. In a newly reported porphyrin-centered dendritic complex, porphyrin was activated by fluorescence resonance energy transfer (FRET) from the covalently bonded TPE peripheral chromophores and sensitized the production of singlet oxygen, which is crucial in photodynamic therapy. Lanthanide(iii) ions directly coordinated to proteins, nucleic acids, and biologically relevant chromophores have been shown to act as luminescent biolabels upon TPE sensitization. Here, TPE–EET provides a new way for extending the wavelengths needed for sensitization which may lead to less-harmful and deep-penetrating bioimaging applications. Intrinsic biological fluorophores directly chelated to lanthanide ions generally have low efficiencies of twophoton sensitization owing to their low two-photon-absorption cross sections: for example, the two-photon-absorption cross sections of amino acids such as tyrosine residues in a Tb–protein complex lie between 0.4–7.4 GM in the spectral region of 502–566 nm. This low efficiency may be an inherent restriction for the intrinsic TPE sensitizers. Herein, we report the efficient TPE–EET sensitization of Eu luminescence in the [Eu(tta)3dpbt] complex. The twophoton-absorption cross sections of dpbt in both the free form and when chelated to Eu were measured over the spectral region of 730–830 nm by the use of femtosecond laser pulses: the maximal values are 185 and 157 GM for dpbt and [Eu(tta)3dpbt], respectively. This Eu complex displays both efficient two-photon sensitization and high-purity red emission. In Figure 1 we see that the fluorescence excitation spectrum of dpbt in toluene is identical to the one-photonabsorption spectrum. This suggests that the absorption band between 300 nm and 400 nm results mainly from the absorptive transition from the ground state (S0) to the S1 state, which undergoes efficient vibrational relaxation upon one-photon excitation. Importantly, the absorption/excitation and the Scheme 1. Structures of the ligand dpbt and the [Eu(tta)3dpbt] complex.